I have a continent size map and want to create a more detailed local map. To do that, I selected the interesting area of the continent map, exported the selected area. Then, I created a new project and attempted to import the DMF file I just exported. To my surprise, DMF is not a supported format (why?). However, following on the advice of this post:

Thanks for the bug report. I can't immediately say what's gone wrong or how to work around it, since this problem is seldom encountered and hence occurs in code that has not been sprinkled in error checking or debugging statements. I have however reproduced the fault, so time permitting I should be able to look into this in the next week or so. I apologise for the inconvenience caused in the meantime.

To answer your why? question: import of the DMF file format was not originally supported because there was no demand from users; no other program could export that format. Obviously, that argument doesn't cover the case of users wanting to re-import design maps from L3DT, but that happens surprisingly infrequently - two requests in 8 years. Nonetheless, I will put this on the to-do list for the next release.

As it happens, the DMF format doesn't include certain important design map settings (horizontal scale, wrapping flags, climate lists, etc.), which are instead read from the map definition file. This means that the DMF format is not a fully self-contained description of a design map, so is not suitable to be imported by itself. However, in the next release the DMF will finally be replaced by a new data format that includes all relevant information, and this format will permit the export and re-import of the design map. If the fates smile upon my endeavours, I may be able to get a developmental build of this out for testing before Christmas. Otherwise, it should be available early in the new year. Whatever the outcome, I'll post back here when there's something ready to test.

My ultimate objective with exporting part of a design map and then re-importing it is to create continent-scale landmasses for near-orbit rendering (I use Terragen), and then creating a plausibly-similar detail map for ground level renders. Will the new format and features you're working toward support that use case?